Boiling Point at Altitude Calculator
Determine how altitude lowers boiling temperature by accounting for reduced atmospheric pressure and enthalpy of vaporization.
Use 373.15 K (100 C) for water at sea level.
Water near 100 C has DeltaHvap about 40.65 kJ/mol.
Valid for troposphere altitudes (below ~11 km).
Atmospheric pressure
84.56 kPa
Boiling point
368.07 K
94.92 C
How to Use This Calculator
Start with a reference
Enter a known boiling point and pressure for the liquid, typically the sea level values.
Measure or estimate altitude
Provide the altitude above sea level where the boiling point is needed.
Enter enthalpy of vaporization
Use tabulated DeltaHvap near the reference temperature for the substance.
Review the results
The calculator reports both the reduced pressure and the new boiling temperature.
Formula
P(h) = P0 * (1 - Lh/T0)^(gM/(R L))
ln(P2/P1) = (-DeltaHvap / R) * (1/T2 - 1/T1)
Combine the international standard atmosphere with Clausius-Clapeyron to solve for T2. For altitudes above 11 km, an exponential is used.
Example
At 2500 m: P is about 75.1 kPa and water boils near 365 K (92 C) using DeltaHvap = 40.65 kJ/mol.
Full Description
Atmospheric pressure drops with altitude, reducing the temperature needed for vapor pressure to equal ambient pressure. The Clausius-Clapeyron relation links this pressure change to boiling point via enthalpy of vaporization.
This tool uses the standard atmosphere model up to the tropopause to estimate pressure, then adjusts the boiling point accordingly. It is most accurate within the troposphere and for pure substances.
Frequently Asked Questions
Why does boiling point drop with altitude?
Lower pressure means vapor bubbles form more easily, so less thermal energy is needed to boil.
How accurate is the standard atmosphere model?
It is a good average for the troposphere. Local weather variations can shift pressure by a few kPa.
Can I use Fahrenheit?
Convert Fahrenheit to Kelvin before entering values. The calculator assumes SI units internally.
What happens above 11 km?
The temperature gradient changes. The calculator switches to an exponential pressure decay beyond that height.
Do I need to adjust DeltaHvap with altitude?
DeltaHvap varies slightly with temperature. Using a value near the reference boiling point is sufficient for estimates.